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Dissociative substitution describes a pathway by which compounds interchange ligands. The term is typically applied to coordination and organometallic complexes, but resembles the Sn1 mechanism in organic chemistry. This pathway can be well described by the cis effect, or the labilization of CO ligands in the cis position. The opposite pathway is associative substitution, being analogous to Sn2 pathway. Intermediate pathways exist between the pure dissociative and pure associative pathways, these are called interchange mechanisms.〔Basolo, F.; Pearson, R. G. "Mechanisms of Inorganic Reactions." John Wiley: New York: 1967. ISBN 0-471-05545-X〕〔R. G. Wilkins "Kinetics and Mechanism of Reactions of Transition Metal Complexes," 2nd Edition, VCH, Weinheim, 1991. ISBN 1-56081-125-0〕 Complexes that undergo dissociative substitution are often coordinatively saturated and often have octahedral molecular geometry. The entropy of activation is characteristically positive for these reactions, which indicates that the disorder of the reacting system increases in the rate determining step. ==Kinetics== Dissociative pathways are characterized by a rate determining step that involves release of a ligand from the coordination sphere of the metal undergoing substitution. The concentration of the substituting nucleophile has no influence on this rate, and an intermediate of reduced coordination number can be detected. The reaction can be described with k1, k-1 and k2, which are the rate constants of their corresponding intermediate reaction steps: Normally the rate determining step is the dissociation of L from the complex, and () does not affect the rate of reaction, leading to the simple rate equation: : However, in some cases, the back reaction (k-1) becomes important, and () can exert an effect on the overall rate of reaction. The backward reaction k-1 therefore competes with the second forward reaction (k2), thus the fraction of intermediate (denoted as "Int") that can react with L' to form the product is given by the expression , which leads us to the overall rate equation: : When () is small and negligible, the above complex equation reduces to the simple rate equation that depends on k1 and () only. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dissociative substitution」の詳細全文を読む スポンサード リンク
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